In a known modern automotive internal combustion engine, a main suction passage is juxtaposed with a by-pass line for by-passing a throttle valve in order to keep the engine speed during idling operation (i.e., idling speed) at an optimum target speed depending on the engine load, and the by-pass line area is varied by means of an ISC valve (idle speed control valve) of the stepping-motor type. According to the engine of this type, an ECU (electronic control unit) used as a control device is supplied with load information for an air conditioner compressor, alternator, power steering pump, etc., besides operating information, such as the engine speed, cooling water temperature, etc. Based on these pieces of information, the ECU performs arithmetic processing and the like, and settles the drive step volume of the ISC valve which realizes a proper idle speed.
In general, speed feedback control is used as a mode of drive control for the ISC valve. If the idle speed is deviated from its target value (reference idle speed), according to this control, an intake correction value for eliminating the deviation is obtained by computation or map retrieval, and the ISC valve is driven to the opening or closing side in accordance with the intake correction value, whereby a stable idle speed can be obtained despite the variation of load. In recently developed speed feedback control, the speed feedback control is temporarily terminated when the air conditioner compressor, alternator, etc. are started or stopped, and is switched over to open-loop control such that only load correction is effected for a predetermined time, in order to prevent a control delay or overcontrolling when the load changes suddenly.
Normally, the aforesaid switching from the speed feedback control to the open-loop control is achieved as the load changing rate exceeds a predetermined threshold value. In doing this, the load changing rate is obtained by detecting the load with every predetermined sampling period, and dividing the resulting deviation by the sampling period. Thus, the control response characteristic can be improved by shortening the sampling period and lowering the threshold value. In this case, however, if the detected load involves a minute fluctuation or noises, the load changing rate is naturally liable to change suddenly, sometimes causing wrong operation.
If the sampling period and the threshold value are lengthened and increased, respectively, on the other hand, there is no possibility of wrong operation, and stable control can be effected. In this case, however, the value of the load changing rate is too small to ensure control switching if a large load change, if any, converges within the sampling period. Thus, the control response characteristic and the control stability are in tradeoff relation, and cannot be reconciled to a high degree. In order to prevent an engine stall or the like when the load changes suddenly, therefore, the reference idle speed must inevitably be set at a pretty high value, so that the fuel cost performance is worsened. In the case of an idle-cylinder engine in which the operation of some cylinders is stopped in a low-load state, moreover, idling becomes particularly unstable when the load fluctuates, causing a substantial vibration.
The object of the present invention is to provide an idle speed control method and apparatus for an internal combustion engine, ensuring stable idling despite a fluctuation of engine load.